Buildings have traditionally been engineered to withstand the environmental forces expected in the region in which they reside. For example, a building in California might be designed to withstand earthquakes while a building in Florida might be designed to withstand hurricanes. However, the stresses a building might be expected to withstand in its lifetime are no longer limited to natural phenomena, but now include the possibility of acts of terror. As such, protection against the forces associated with explosions is necessary for some buildings which might be the targets of such acts of terror.
One common defense against explosions has been to prevent access to the target building. One way to prevent access to a building is to increase the setback from the building. The setback is measured as the distance of access to a building. Setback can be effective since explosive force is related to distance—the greater the distance an explosive force travels, the lessor the force experienced by the building during a blast. For example, some high-profile buildings have barriers erected around the perimeter of the building which prevent automobiles from driving up to the building. Since automobiles are often used to carry explosives, placing barriers around a building can help to minimize the effect if the explosives are detonated. Unfortunately, in some cases adequate setback is not possible. For example, historical buildings, especially historical government buildings, are often prime targets for terrorist activities, and often are positioned immediately adjacent a street. As such, in some cases setback alone is insufficient to protect a building.
The most vulnerable part of a building during a blast is typically the windows, as the glass on windows will shatter during an explosion, the sharp pieces becoming high-speed projectiles in the building. It is costly to retrofit an existing building with explosion-resistant glass, and typically involves expensive reinforcement and reengineering of the window sills. Such a retrofit typically requires use of the building be suspended or altered during construction.
Previous force-resistant panels have been provided which are anchored to the window sill at the upper and lower ends, leaving the middle portion of the window movable during a blast. In such designs, as viewed in vertical cross-section, the panel bends in a parabolic shape during a blast. In such designs, a parabolic-shaped stop is built on the window sill to catch the panel as it bends. Such designs are insufficient due to the cost and complexity of building the parabolic stop and the stresses related to the bending of the window. Such designs are also insufficient due to the stresses on the building at the sills where the panels are anchored. Further, deformation of the panels during a blast can be non-uniform, causing the panel to improperly engage the stop and come free during a blast. An improved blast panel is needed.